This application claims the priority of Japanese Patent Application No. 5-141566 filed on May 20, 1993 which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a signal processing circuit of a simultaneous electronic endoscope apparatus and, more particularly, to the structure of processing of a video signal in a simultaneous electronic endoscope apparatus provided with a color filter for each pixel.
2. Description of the Related Art
An electronic endoscope apparatus is an apparatus for observing a body cavity such as an alimentary canal and a capillary of a structural body through an electronic endoscope provided with a CCD (Charge Coupled Device) as a solid-state image sensor at the end portion thereof. As such an electronic endoscope apparatus, a simultaneous electronic endoscope apparatus is known. In this apparatus, pixel signals are extracted through a color filter provided for each pixel and, video signals such as a luminance signal and a color difference signal are formed from the pixel signal.
FIG. 7(A) shows the arrangement of color filters which are provided on the surface of a CCD 1 in a simultaneous electronic endoscope apparatus. As shown in FIG. 7A, four kinds of filters, namely, Mg (magenta), Cy (cyan), G (green) and Ye (yellow) filters for the respective pixels are disposed in a predetermined positional relationship. Color filters arranged as shown in FIG. 7(A) as one pattern are repeatedly provided on the entire image pickup area. The signals obtained in the respective pixels of the CCD 1 through the color filters are alternately output in the form of the mixtures (sums) of the upper and the lower signals Cy G (A), Ye+Mg (B), . . . on a first horizontal scanning lines ODD1 of an odd field, and in the form of the mixtures (sums) of the upper and the lower signals Cy+Mg (C), Ye+G (D), . . . on a second horizontal scanning line ODD2, as shown in FIG. 7(B). Similarly, the signals are repeatedly output thereafter in the form of the sums to the last horizontal scanning line. In the even fields, the mixture signals of the upper and the lower signals signals are also alternately output as the signal of horizontal scanning lines EVEN1, EVEN2, . . .
From the outputs of the CCD 1 obtained in this manner, a luminance signal Y, and color difference signals, (R (red)--Y) and (B (blue)--Y) , for example, are calculated by an operation circuit such as a color separation circuit, and these signals are converted into a composite video signal by an encoder or the like and output to a monitor, thereby displaying the image of the object of observation on the monitor.
The above-described conventional charge coupled device for an electronic endoscope, however, suffers from a problem of the deterioration of the picture quality due to the difference of the saturation property in the CCD outputs of the mixture signals which is caused by the difference of the sensitivity in the color filters. When the signals shown in FIG. 7 are processed, if the mixture signal Cy+G on the horizontal scanning line ODD1 is assumed to be A, Ye+Mg to be B, the mixture signal CY+Mg on the horizontal scanning line ODD2 is assumed to be C and Ye+G to be D, the saturation properties of these mixture signals in the wavelength range on the red side are as shown in FIG. 8. Since the sensitivities of Mg (magenta) and Ye (yellow) are higher than those of Cy (cyan) and G (green), the mixture signals B, D, C and A reach the saturation point E when the quantities of incident light are P1, P2, P3 and P4, respectively. In this case, if the outputs of the mixture signals exceed the saturation point E, the luminance signals on the horizontal scanning lines become different from each other, so that scanning lines loom around a whitish saturation region 100, thereby deteriorating the picture quality.